by Lawrence G.
Desmond, William A. Sauck, James M. Callaghan, John Muehlhausen
and Kristen Zschomler

August 1, 1993University of ColoradoBoulder, ColoradoU.S.A.

Abstract

A geophysical survey
of selected areas of the Great Plaza and Great Ball Court at Chichen Itza
was carried out February 4-10, 1993 using ground penetrating radar, and
electrical resistivity. Ground penetrating radar was used to survey
more than six kilometers of transects, and 144 Megabytes of data was recorded
by the computer. The electrical resistivity of 30 meter squares in
both the Great Plaza and Great Ball Court was measured in 1,800 discrete
locations. Analysis of the data from the ground penetrating radar
survey has provided topographic contours of the bedrock from radar reflection
time depth estimates, and identified discrete natural or cultural features.
The resistivity method detected conductivity variations in the survey areas
with some effects due to contemporary cultural features.

"It seems that I am in the landof the gods--the rising sun comesup very big and hot looking throughthe thin cool early morning mistthat hovers over these pyramidsand temples with the dawn..."Georgia O'Keeffe at Chichen Itza.February 23, 1951.

Introduction

Plans for a ground penetrating radar project
originated in 1977 with discussions between Lawrence Desmond, an instructor
of archaeology at Foothill College, and Lambert Dolphin and Roger Vickers,
geophysicists with the Radio Physics Laboratory of nearby SRI International
in Menlo Park, California. Both Dolphin and Vickers had extensive
experience using ground penetrating radar, and in 1974 they had participated
in a joint project in which SRI International had collaborated with Egypt's
Ain Shams University, and the Egyptian government's Organization of Antiquities
to survey the pyramids at Giza. In 1977 they returned to continue
their survey using electrical resistivity, acoustical sounding and aerial
infrared imagery. The main problem they faced at Giza in the use
of ground penetrating radar was high radio frequency losses due to the
type of limestone in the area which made radar a poor choice for a remote
sensing survey of that location.

Chichen Itza, which sits on limestone bedrock,
seemed to Desmond to be an ideal location to carry out a ground penetrating
radar survey, but Dolphin and Vickers cautioned that without analysis of
the limestone it was not possible to predict the effectiveness of radar.
And, such an analysis would not provide a complete answer since the quality
of limestone bedrock might vary in different areas. They suggested
samples be gathered, tested and if the results looked promising attempt
a survey.

In 1985, four limestone bedrock samples
from the vicinity of Chichen itza were analyzed by SRI International.
Thomas Yetter, an assistant to Dolphin at the Radio Physics Laboratory,
made the radio frequency loss measurements. Early in 1986, in a report
of their analysis, Dolphin stated, "What you can see...is that the limestones
are quite low in losses even when wet and the losses are not severe at
high frequencies where resolution is excellent. We are greatly encouraged
to see that ground penetrating radar appears so ideally suited for probing
into structures and into the ground at Chichen Itza" (Dolphin 1986).

Encouraged by the Dolphin-Yetter report,
the Instituto Nacional de Antropologia e Historia (INAH) was contacted
to determine when a project might be scheduled. At the same time
efforts were made to find a geophysicist who had equipment available and
would be willing to carry out a survey at Chichen Itza.

Early in 1991, at a meeting of the American
Association for the Advancement of Science, William Sauck was asked by
Desmond if he would be interested in carrying out a ground penetrating
radar survey at Chichen Itza. Sauck agreed enthusiastically, and
asked to be sent the Dolphin-Yetter limestone analysis. Sauck, a
geophysicist with the Institute for Water Sciences at Western Michigan
University, had experience applying geophysical techniques to archaeology
in tropical environments and so was an excellent choice for the project.
In his opinion a survey at Chichen Itza would have a good chance of success.

In the spring, Desmond, James Callaghan,
director of the Fundacion de la Universidad Autonoma de Yucatan, A.C.,
Archaeologist Agustin Pena Castillo, co-director with Archaeologist Peter
Schmidt of INAH's Proyecto Chichen Itza, and Alfredo Barrera Rubio, regional
director for INAH in Yucatan met in Merida to discuss the project.
It was agreed that the project would come under the direction of Proyecto
Chichen Itza, and the Great Plaza and associated Great Ball Court would
be the focus of our work. There were a number of other project locations
suggested such as the area north of the Xtoloc Cenote, and the vicinity
of the High Priest's Grave where additional chambers of known caverns might
be located. But, a greater priority was given to a survey of the
Great Plaza and Great Ball Court areas in order develop a contour map of
the bedrock topography and determine the extent of terracing. Also,
any remains of buried structures or other cultural features located during
the process of subsurface mapping might add to our understanding of the
architectural chronology at Chichen Itza. We set the date of our
work to be the first two weeks in February 1992.

But, serious illness in Dr. Sauck's family
caused us to postpone the project, and INAH agreed that its schedule allowed
for the work to be carried out in February 1993. Late in 1992 with
the assistance of James Callaghan, who had taken on the responsibility
of field director, a number of last minute details were arranged with INAH
such as authorization for the temporary import of equipment, project transportation,
and official letters.

Since the project was part of INAH's Proyecto
Chichen Itza we wanted to report our findings as quickly as possible to
INAH archaeologists so that they could incorporate them into their on-going
project. To provide a report of activities before Desmond returned
to the University of Colorado, survey maps were prepared daily by John
Muehlhausen and Kristen Zschomler, and notes were made by Sauck of the
location of anomalies from the ground penetrating radar video display.
In that way we were able to make a presentation at the INAH office in Merida,
and provide a written summary of our work before departing.

Background

Ground penetrating radar uses high frequency
radio waves which reflect from both natural and cultural subsurface features,
and provides a two dimensional cross-sectional presentation which is used
to evaluate details of subsurface conditions. Current ground penetrating
radar systems are comprised of a central processing unit and software which
controls the capture and modification of radar data in RAM memory, and
then transfers the data to a digital cassette. A video display provides
an operator with real time monitoring of the subsurface as the antenna
moves over it. Antennas come in a number of sizes and the choice
comes after some testing at an individual site. From the small amount
of data available to him on the subsurface of Chichen Itza, Sauck decided
on two antennas: a 500MHz which is used for exploration down to two to
three meters from the surface, and a 100MHz antenna for deeper surveying.

Ground penetrating radar systems also allow
investigators to review survey data in color on a video display by recalling
data from computer memory and tape or by printing the data out as hard
copy in color or black-and-white. The system allows for a number
of adjustments to maximize the radar antenna output to conform to a particular
geophysical setting, provides further modification of captured data by
filtering out external radio frequency noise and electronic artifacts generated
by the equipment itself, and the color pattern of radar reflections can
be changed at the monitor or on hard copy. These adjustments allow
for a variety of analytical approaches and help in the visual identification
of subsurface features. The plotted radar data is presented in two
parts on one sheet in this report with field data below and processed data
above (Figures 7, 8,
9,
10,
11).

The instrument used in this project is
called a Subsurface Interface Radar System-10 and was manufactured by Geophysical
Survey Systems, Inc. It was loaned to the project by the Institute
for Water Sciences of Western Michigan University (Photos 1,
and 2).

To survey the subsurface of the Great Plaza
and Great Ball Court the central processing unit and monitor were mounted
in the back of a pickup truck loaned by INAH, and Sauck controlled the
unit from there (Photos 3,
4,
5).
The radar was run on twelve volt auto batteries, and the antenna, connected
to the central processing unit by a 15 meter cable, was dragged behind
the moving truck by archaeologists along previously established transect
lines (Photo 6).

To layout the transect lines, a datum was
set at the base of the northeast corner of the Castillo Pyramid and lines
were laid out in the Great Plaza at 10, 5 and 2.5 meter intervals.
Most of the survey was carried out in 10 meter intervals but in areas of
special interest 5 and 2.5 intervals were used. Flags marked each
10 meters along a transect so that the location of the antenna along a
line could be electronically marked by a switch operated by the archaeologist
pulling the antenna (Photos 7
and 8). Thus, when the
data from a transect is played back or printed, each 10 meters is marked
on the video screen or on the print out. This provides the geographical
control used in mapping bedrock contours and locating in three dimensional
space unidentified subsurface features for further investigation.

In the Great Ball Court a similar system
was used with two north-south transects the length of the court (Photo
9),
two east-west transects in the center and one east-west in each end zone
(Photo 10). A single
transect was run from the Platform of Venus to the Cenote of Sacrifice
(Photo 11).

Electrical resistivity is a well tested
technique used to measure subsurface conductivity (how difficult or easy
it is for an electrical current to travel through the ground). Different
kinds of soil have different resistances to the flow of current.
A loose, dry, sandy soil will probably have a higher resistivity than a
moist, silty soil with a high concentration of ions, and a cavity will
have very high resistivity.

In archaeology, measurements are usually
taken systematically in the area to be investigated so that visual identification
of soil change boundaries can be made using a dot-density computer generated
printout. The data this technique provides to the archaeologist is
one of several sources used to develop a research strategy and make decisions
concerning excavations.

The type of resistivity instrument used
in this project is called a RM15 Resistance Meter and was made by Geoscan,
Inc. (Photo 12).
Measurements were made by John Muehlhausen of the Institute for Minnesota
Archaeology in Minneapolis which loaned the instrument to the project.
The RM15 runs on eight "AA" 1.5 volt batteries and has two electrodes spaced
at one meter that are attached to a light weight frame along with a portable
computer (Photo 13).
Two other electrodes which provide a return for the current flow and a
voltage reference are connected to the computer with a long wire, and then
inserted into the ground at distance of 30 meters from the area of survey.
The electrodes are arranged in what is called a pole-pole array.

In operation, the electrode/computer unit
is moved to each location to be measured and the electrodes are inserted
into the ground (Photo 14).
The computer senses when the electrodes have been inserted sufficiently
for electrical contact, and then it measures and records the applied current
and the voltage difference at that location.

At Chichen Itza, two 30 meter squares with
data points at one meter grid intervals were surveyed using resistivity.
One was at the northeast corner of the Castillo Pyramid (Photo 15),
and the other at the center of the Great Ball Court (Photos 16,
and 17).

Software developed by Geoscan, called Geoplot,
calculates the resistivity for each set of field readings in the computer
and plots the data with a dot-density pattern. The darker the area
on a plot the more resistivity recorded, and the lighter the more conductive
the area.

The scope of the research

The following field objectives and priorities
were carried out. For ground penetrating radar: 1) Survey the subsurface
of the Great Plaza to the east and north of the Castillo Pyramid (Photo
18),
2) survey north-south to the west of the Castillo Pyramid with one transect,
and to the south of the pyramid east-west with two transects, 3) survey
the Great Ball Court with two transects north and south, two transects
east and west in the center, and one east-west transect in each end zone,
4) survey the center of the sacbe from the Platform of Venus to the Sacred
Cenote with one transect, and 5) survey the area five meters from the base
of the north and west side of the Platform of Venus (Photo 19)
to detect a previously noted floor one meter beneath the surface.
The floor had first been drawn by Augustus Le Plongeon in a profile (Photos
20
and 21) of his excavation
of the Platform of Venus in 1884, and then noted again in 1980 by archaeologist
Peter Schmidt during trenching for the "sound and light" installation (Photo
22).

Electrical resistivity examination of two
areas was carried out: A 30 meter square was surveyed in the center of
the Great Ball Court, and a 30 meter square at the northeast corner of
the Castillo Pyramid.

The fixed remote probes of the resistivity
instrument were placed 30 meters from the grid edge and the moveable probes
were set one meter apart on the frame. With the one meter probe separation,
readings to an approximate maximum depth of 1.5 meters were made.
Resistivity measurements were then taken at one meter intervals inside
each grid for a total of 900 readings per grid.

Finally, an important component of this
project was to test the effectiveness of ground penetrating radar and resistivity
for exploration of the subsurface at Chichen Itza, and, successful or not,
to provide technical information about the project.

Great Plaza1. North from the Castillo Pyramid to
the Platform of Venus, and west from the Temple of the Warriors to north-south
line 70W. 2. East from the Castillo Pyramid 50 meters (50E) toward
the Temple of the Warriors. (Photo 23)3. West from the Castillo Pyramid:
One line north-south, 70W, and lines east-west 10N and 20N to the east
wall of the Great Ball Court. 4. South of the Castillo Pyramid two
transects east-west 65S and 70S. (Photo 24)

Platform of Venus 1. Area west and north of the northwest
corner: three transects south to north to detect known floor approximately
one meter below the surface with an edge about 8 meters from the platform
base.

Great Ball Court 1.Two north-south transects in the
Great Ball Court with the west transect set 10 meters east of the west
wall and the east transect 10 meters west of the east wall. 2. Two transects were made east-west
in the center of the Great Ball Court. One was between the rings
set into the east and west walls and the other 10 meters south and parallel
to that line. 3. One transect was made in each
end zone and was run parallel to the east-west wall in each area.
The north end line was 15.4 meters south of the north wall, and the south
end line was 18 meters north of the south wall.

Sacbe from the Platform of Venus to the
Sacred Cenote. 1. One transect down the center
of the sacbe to about 20 meters before the Sacred Cenote starting at the
northwest of the Platform of Venus.

Great Plaza 1. A 30 meter square area from the
datum in northeast corner of the Castillo Pyramid. From datum to
15N and 15S, and to 30E.

Great Ball Court 1. A 30 meter square area wall-to-wall
and 15 meters north and 15 meters south from the rings in the east and
west walls.

Ground penetrating radar
survey results

The first step in analysis of ground penetrating
radar data requires the processing of the digital information recorded
from radar reflections by filtration and color manipulation. The
ground penetrating radar data was printed by a color plotter, but has been
reproduced for this report in black-and-white (Figures 7,
8,
9,
10,
11).
The lower portion of the printout is unprocessed field data including both
internally and externally generated noise, the upper section has been processed,
and notations on the plotted data are by Sauck.

Based on radar soundings, Sauck has plotted
the bedrock topography contours of the area north of the Castillo Pyramid,
east-west transects 39N to 69N, and from the Temple of the Warriors to
north-south transect 70W (Figures 4
and
5). Two way radar reflection
times in nano-seconds (10-9sec) are noted on the contour lines of the subsurface
map. Using a reasonable value (12) for relative permittivity, we
estimate that 23 nano seconds is equal to one meter. Rather than
convert the depths to meters at this time, excavation data will be used
to calibrate the radar reflections to absolute depths in a forthcoming
report .

In the area at 66.5N, 60E, about 10 meters
to the west of the Temple of the Warriors, is a small bedrock mound (Figures
4,
5 and 7).

The subsurface topography to about 80 meters
west of that mound is fairly even, but at 66.5N, 10W is evidence of the
east side of an oblong mound which is most likely of bedrock and measures
about 20 meters east-west, and 30 meters north-south (Figures 4,
5,
and 8).

In addition to the relatively even bedrock
topography from the Temple of the Warriors subsurface mound to a point
80 meters west, there also appear to be few cultural features. Beyond
80 meters west (66.5N, 10W) and to the west of the Platform of Venus, stratified
fill, possibly floors, and other cultural features become abundant on the
upper part of the radar sections (Figure 8).

At 66.5N, 30W or 100 meters west of the
Temple of the Warriors is a near-surface disturbance. It appears
to be within two meters of the surface and has given a very strong radar
reflection. Excavation is needed to determine if it is a natural
or cultural feature (Figures 4, 5,
and 9).

The transect 60W, from the northwest corner
of the Castillo Pyramid, to 59N shows a number of strong reflections which
we think are floors between 10N and 20N, and between 50N and 57N (Figures
10
and 11).

The floor excavated and drawn by Le Plongeon
in 1884, and noted by archaeologist Schmidt in 1980 (about one meter below
the surface in the area to the northwest of the Platform of Venus), was
not detected using the 100 MHz radar antenna. It is most likely that
the mixed fill of the terrace did not allow the floor to be distinguished.
Another antenna with a higher frequency might be able to locate it, but
only if there is a contrast in electrical properties between the floor
and the fill.

Bedrock topography and other information
from survey transects of the Sacred Cenote sacbe, the Great Ball Court
and other locations will be available in a forthcoming report.

Resistivity survey results

Data captured by the Geoscan RM15 was transferred
to a desk top computer and printed using Geoscan Geoplot software to produce
dot-density plots of each survey grid (Figures 12
and
13).

A 30 meter square area at the northeast
corner of the Castillo Pyramid was surveyed from 0N to 15N, 0E to 30E,
0N to 15S (Figures 6 and 12).
An area of low resistivity (white), noted at about 15 meters east, begins
at the north boundary of the grid. It is about nine meters in width
and becomes more resistant (darker areas) 15 meters south. Another
area of low resistivity, 2N to 15S, cuts diagonally into the grid (Figure
12).

The low resistivity is likely due to highly
compacted and chemically very conductive fill, but these resistivity anomalies
(high or low) are subtle and it is difficult to know whether they represent
natural or cultural features without a general resistivity survey of the
Great Plaza.

Within the Great Ball Court a 30 meter
square was surveyed using resistivity (Figures 6
and 13). The grid was set with
0N and 0E under the ring on the west wall of the court.

The most clearly defined area of low resistivity
is a one-two meter wide area beginning at 15N, 0E (the northwest corner
of the grid) which cuts diagonally to 15S, 14E. A second diagonal,
about the same width, begins at 14S, 6E and cuts through the grid to about
6S, 30E or across the bottom of the grid to the east wall. These
low resistivity areas appear to be the result of filled excavation trenches.
Also in the area surveyed there are a number of other areas where changes
in resistivity are subtly represented by light and dark in the plot and
will remain uninterpreted until further fieldwork and analysis is carried
out.

Conclusions

This project has demonstrated the successful
use of ground penetrating radar at Chichen Itza. We recommend an
additional survey to provide topographic contours of the total subsurface,
and to map the limits of the Great Plaza. The responses of different
antennas should be studied on site to determine the optimal frequencies
for use at Chichen Itza, and at the same time examination of radar reflection
signatures should be made systematically to enable the identification of
natural and cultural features in the fill.

Electrical resistivity testing was carried
out in a very limited area, but the survey results have shown the technique
to be well adapted to the environment of Chichen Itza. No less than
a survey of the whole Great Plaza and Great Ball Court should be made to
allow for analysis of the total resistivity pattern.

Acknowledgements

Fieldwork for the Chichen Itza Geophysical
Survey Project was more than ten years in preparation, and during that
time contributions, both financial and in-kind, were made by a great
number of institutions and individuals. As with many archaeological
activities, this project was multi-disciplinary and international in scope,
and required close coordination among varied institutions and individuals.
The authors would like to extend their most sincere thanks to everyone
who helped to make this project a success.

Our colleagues at INAH deserve a special
note of thanks for their very important contributions to the project.
Alfredo Barrera Rubio, director of INAH-Centro Regional Yucatan, and Jose
Huchim Herrera coordinator of the archaeology section arranged our fieldwork
plans and other official requirements, and Proyecto Chichen Itza co-director
Agustin Pena Castillo assisted us in the development of the research plan.
At Chichen Itza, site administrator Feliciano Salazar L. made us welcome
and arranged that our work could be carried out efficiently and quickly.

We also would like to thank the Institute
for Water Sciences of Western Michigan University for financial assistance
and the use of its ground penetrating radar system, and acknowledge the
generosity of Clark Dobbs and the Institute for Minnesota Archaeology for
the loan of that organization's resistivity equipment. Both institutions
continue to provide support for post-fieldwork data analysis and map making.
Special thanks to Arthur Dunkleman, president of the Institute of Maya
Studies, who, with great dedication, worked with Mexicana Airlines and
Mexico's Secretary of Tourism to arrange for funded flights to Merida for
Desmond and Sauck. He also arranged through Jose de Lima of Tourism
for a grant from the Secretary of Tourism to cover field expenses, and
with Fernando Barbachano, Jr. for complimentary rooms at the Mayaland Hotel
at Chichen Itza. To both Jose de Lima and Fernando Barbachano, Jr.
we extend our appreciation. Thanks also to Russell Etling, director
of the Miami Museum of Science, for assistance in arranging for the deposit
and disbursement of project funds from Mexico. To Ernie Marc of the
Graves Museum of Archaeology, many thanks for pitching in to help with
the fieldwork and for wonderful video documentation of our work.
And, at the University of Colorado thanks to master photographer Larry
Harwood and illustrator Diane Lorenz for the many hours spent preparing
our illustrations for publication. Our appreciation and thanks to
David Carrasco, director of the Mesoamerican Archive and Research Project
of the University of Colorado, for providing a post-fieldwork grant for
completion of the project report.

An important contribution to the success
of the project was made early on by Lambert Dolphin, Roger Vickers and
Thomas Yetter of the Radio Physics Laboratory of SRI International.
They spent many hours explaining the potential use of radar to survey the
subsurface at Chichen Itza, and provided analysis of the limestones collected
by our colleague Elizabeth Baquedano of the Universidad Nacional Autonoma
de Mexico.

We would also like to acknowledge direct
financial support or in-kind contributions over the years from the following
research institutions: the University of Minnesota, the California Academy
of Sciences, the Foundation for the Advancement of Archaeology through
Collaboration in Technology (FAACT), the Mesoamerican Archive and Research
Project of the University of Colorado,Fundacion de la Universidad Autonoma de
Yucatan, A.C., and Stanford Research Institute International.

Finally, my personal thanks for terrific
work and great friendship to geophysicist Bill Sauck, and archaeologists
James Callaghan, Kristen Zschomler and John Muehlhausen. Lawrence
G. Desmond, Boulder, Colorado. (Photo 25)